The present invention relates to heat flow meter instruments for testing thermal properties of materials including thermal conductivity and heat capacity. More particularly, the invention relates to a closed loop heat flow meter instrument that utilizes thermoelectric devices for controlling temperature.
Thermal properties, such as thermal conductivity and heat capacity, are important physical properties of solids. Heat flows through a solid that has a temperature gradient across its volume. The thermal conductivity of a specimen can be measured directly by measuring the heat flux resulting from a known temperature gradient across a known thickness. A one-dimensional form of the Fourier heat flow relation is frequently used to calculate thermal conductivity under steady-state conditions: ##EQU1## wherein k is thermal conductivity, Q is a heat flow per a unit surface area (heat flux), .DELTA.T is a temperature difference over the thickness .DELTA.X. A standard instrument measures the thermal conductivity of a specimen located between two flat plates by maintaining the plates at known, but different temperatures. As heat flows through the specimen from the hot side to the cold side, a heat flux transducer measures the amount of heat transferred. A thermocouple measures the temperature of each plate.
In some instruments, the two plates are heated (or cooled) separately. Usually, each plate is heated by an electric heater, such as an etched foil heater or a wire heater, powered by an electric power supply. The plate may also be thermally connected to a heat sink with a circulating coolant. The desired plate temperature is reached by balancing the action of the heater and the cooling system. The balancing commonly causes pulsation of the temperature, and thus the plate provides pulsating amounts of heat to the specimen. To reduce the pulsating effect, a thermal buffer is frequently placed between the plate and the heat flux transducer. Therefore, it may take a relatively long time to achieve a thermal equilibrium at a desired temperature. Alternatively, each plate may be heated or cooled by regulating temperature of the fluid circulating in the heat sinks. This type of temperature regulation may increase the cost of operation. In either case, the output of the heat flux transducer and of the thermocouples is monitored until variations in temperature subside and steady-state heat conditions exist. Then, the instrument measures the thermal conductivity. The measurements are usually performed according to standard testing methods, such as, C 518 or C 1045 methods published in Annual Book of ASTM Standards.
Furthermore, a conventional electric heater may introduce a significant error to the measured data. Some instruments use a heater powered by an AC power supply. The AC signal introduces AC noise into the system. This noise affects detection of the sensor signals since they are at the microvolt levels. Therefore, to provide accurate data, the sensors may need AC shielding.
There are other methods that do not require the steady-state conditions. The thermal conductivity coefficient may be measured by so called thermal diffusivity methods and quasi-stationary methods. The thermal diffusivity methods (e.g., hot wire method, flash method) determine the thermal conductivity coefficient by indirectly measuring the time of flight of a heat pulse across a layer of the specimen. The thermal diffusivity is the ratio of the thermal conductivity, at an average temperature of the specimen, and the heat capacity. The quasi-stationary method measures usually the sum of temperature differences between the two flat surfaces of the specimen, as the heat is conducted toward the colder plate. This method assumes a linear temperature distribution inside the specimen which is an approximation of the actual behavior.
There is a need for a fast, efficient and highly accurate heat flow meter instrument, which has a relatively small size.